1. Field of the Invention
The present invention relates generally to methods and apparatus for regulating print density in an ink-jet printer and more particularly to such a method and apparatus which utilizes an optical sensor for measuring printed line width.
2. Description of the Related Art
Ink-jet printers include a print cartridge having a plurality of nozzles which can print rows of dots. Print media, such as paper, moves along a media scan axis beneath the nozzles which fire ink therefrom to print images on the paper. In some cases, the print cartridge is mounted on a carriage for bidirectional movement across the paper orthogonal to the axis of media movement. In others, the print cartridge is as wide as the print media with the only movement during printing being that of the paper relative to the cartridge.
As used herein, the term Y-axis refers to the axis of paper movement and the term X-axis refers to an axis which is in the same plane and at 90.degree. to the Y-axis. For a printer having a movable print carriage, the carriage moves back and forth along the X-axis. The separation of ink-jet nozzles on the print cartridge in the X-axis direction typically corresponds to the desired resolution (e.g., 1/300th of an inch for 300 dot per inch (dpi) resolution). Resolution along the Y-axis is determined by the frequency of ink-jet nozzle firing and by the speed of paper movement along the Y-axis. To obtain 300 dpi resolution at a frequency of nozzle firing of 3.6 kilohertz, paper must move along the Y-axis under the print cartridge at 12 inches per second.
A typical ink-jet print cartridge includes a plurality of nozzles each having an associated resistor therein. A supply of ink feeds each of the nozzles. When voltage is applied across selected ones of the resistors, the resistor heats ink in the nozzle and ejects a drop of ink from the end of the nozzle and onto the paper moving beneath the print cartridge.
Most prior art ink-jet print cartridges are designed to eject a drop of substantially constant volume for varying voltage pulse energies applied to a nozzle resistor. In other words, the width and magnitude of a voltage pulse applied to a nozzle resistor does not have a substantial effect on the volume of a drop of ink ejected from the nozzle.
There is a prior art patent, U.S. Pat. No. 4,339,762 to Shirato et al., for a liquid jet recording method in which resistors in a print cartridge are designed so that the volume of a drop of ink ejected from the nozzle varies in response to the voltage pulse energy applied to the resistor. Thus, the diameter of a dot of ink from a nozzle which strikes the print media can be varied by varying the voltage pulse energy applied to the nozzle resistor. Therefore, the width of a line printed by such a printer can be made to vary by varying the energy of the voltage pulses applied to the nozzle resistors. This is true of a line comprising a single row of dots generated by ink drops ejected from a corresponding row of nozzles and of a wider line comprised of a plurality of such rows printed adjacent one another.
The size of a printed dot may also vary depending upon several other factors. Different types of paper absorb the ink differently. In some cases printing is done on a polyamide sheet which does not absorb ink at all and thus produces a very large dot and correspondingly wide lines. In addition, ink-drop volume can vary depending upon the ambient temperature and humidity thereby varying the size of the dot made by the drop.
In a 300 dpi printer, the minimum width of a line made up of a single row of printed dots is approximately 120 microns. As noted, variations in print media and ambient temperature and humidity can create variations in the dot size and therefore the width of a line. It would be desirable to control print density by changing dot size and/or by varying the location of dots printed on the paper to maintain resolution.